1. Field of the Invention
The present invention relates generally to heating systems and, in particular, to a heater and control apparatus that includes a visual temperature indicator and that is suitable for maintaining a proper temperature in a liquid environment such as an aquarium, horse water tank, dog, cat or other pet bowl, terrarium water container, fish or garden pond and other similar usages.
2. Description of the Related Art
A variety of heating systems have been developed to maintain liquids at a proper temperature. For example, heating systems have been developed to maintain the water in aquariums at a proper temperature, typically between about 68.degree.-82.degree. F. for tropical fish and/or various aquatic plants. Generally, such heating systems include an electrical heater element interconnected to a thermostat control device such as a bimetallic temperature sensor/switch unit. The control device usually includes a temperature setting controller, such as a control knob, and a sensor intended to provide aquarium water temperature feedback. The control device is therefore supposed to compare the aquarium water temperature to a desired temperature and control operation of the heating element in response to the comparison.
Common types of aquarium heating systems include the "test tube" systems. In the test tube systems, the heater element is housed within a test-tube shaped casing, which is usually made of glass. The heater element heats the air inside the casing such that heat is transferred from the heater element, by way of convection to the air and casing, then via conduction to the water in the aquarium. Another type of aquarium heating system, hereinafter referred to as a mat system, employs a heater element that is contained within a mat of plastic or resinous material. Heat is transferred from the heater element to the water by way of conduction through the mat.
A problem associated with known aquarium heating systems is the tendency of the heater elements or casings to break. Test tube casings, which are typically made of glass, may break due to handling or thermal shock from rapid temperature changes, e.g., due to being heated and then submerged in cold water. Although plastic casings for test tube heating systems have been proposed, such casings fail to adequately protect the heater element which typically comprises a fragile, narrow gauge wire, under certain circumstances. Further, it has been found that the plastic serves as a barrier to efficient heat transfer due to its inherent poor thermal conductivity. Mat systems also fail to adequately protect the heater element under certain circumstances. For example, a blow to the mat can result in the heating element being severed and thereby rendered inoperable.
Another problem associated with known aquarium heating systems is that they are relatively inefficient in transferring heat from the heater element to the water. In the case of test tube systems, heat is transferred from the heater element to the water by way of air inside the test tube and the structural casing, thus entailing significant losses and thermal time lags. In this regard, it will be appreciated that such casings have a thickness sufficient to meet structural requirements and are commonly formed from materials having relatively poor thermal conduction properties, thereby impairing heat transfer. Likewise, in mat systems, heat is transferred from the heating element to the water by way of relatively thick, structural matting. Although the matting materials utilized in such heaters are sometimes characterized in the literature as having good thermal conductivity properties, in reality the matting materials utilized, which also commonly provide electric insulation, are relatively insulative.
A further problem associated with mat systems is that such systems may experience thermal damage in use. The present inventors have attempted to practice the teachings of the proposed mat systems and have found that systems so constructed tend to melt or burn. It has been determined that this is due, at least in part, to the relatively low heat transfer coefficient associated with the insulative matting material. As a result, heat cannot be carried away from the heating element fast enough to prevent the temperature of the heating element from rising to a point at which it melts or burns. Moreover, such systems have not been found on the market.
A still further problem associated with known aquarium heaters is that such heaters can pose a safety hazard, particularly when misused. In one such referenced system, water temperature feedback control aquarium heaters, the water temperature sensor can become located outside of the water due to a drop in the aquarium water level or other reason. Accordingly, the sensor may provide feedback related to ambient air temperature rather than water temperature, thereby causing the heater to continually heat the water, potentially harming the fish and/or plants in the aquarium.
In addition, in some known aquarium heater systems, the heater element can quickly become heated to dangerous temperatures if the heater element comes out of the water. Such temperatures can result in thermal damage to the heater element and may create a fire hazard.
Yet another problem associated with known heating systems is that conventional dials or gauges utilized to set the desired water temperature, which depend on passive light, such as reflected ambient light, to provide an indication of temperature, may be difficult to read in certain lighting conditions. For example, conventional dials or gauges commonly comprise a scale of temperature markings mounted on a rotatable knob. The desired temperature set point can thus be indicated by rotating the knob to align the marking corresponding to the desired setting with a notch adjacent to the knob. As is readily appreciated, such dials or gauges may be difficult to read when the aquarium is kept in a poorly lit area and/or the aquarium lighting is turned off to simulate nighttime conditions. In addition, the markings of the scale may wear off or otherwise become difficult to read, thereby rendering the temperature setting uncertain.
Known aquarium heating systems are also relatively difficult to assemble at the factory level. Assembly of such systems typically involves many steps wherein various components are positioned and interconnected prior to closing and sealing the heating system casing. As is readily appreciated, such processes are relatively complex and time consuming, thereby adding to assembly costs.
In addition, known aquarium heating systems generally include control circuitry which can become overheated unless adequate heat dissipation is provided. Thus, a thermally conductive link is typically inserted between the heating system casing and elements of the control circuitry to dissipate heat generated when the control circuitry is active. Installing such a link further contributes to the complexity of heating system assembly. In addition, such a link may provide inadequate heat dissipation.
An additional problem associated with known aquarium heating systems relates to providing reliable connections between electrical wires. In constructing aquarium heating assemblies, it is sometimes necessary or desirable to interconnect electrical wires of different thicknesses or gauges. For example, heater elements commonly comprise a narrow gauge wire whereas the wires interconnecting the heater element to other elements of the heating system are usually thicker. Interconnections between wires of different gauges may tend to fail under tension if the wires are simply crimped or soldered together. The problem of failing interconnections is exacerbated where the interconnections are located underwater. Known mechanisms and methods for interconnecting wires of different gauges underwater are expensive and complex.